1. Field of the Invention
The present invention relates to knock detecting apparatus for detecting the presence of knock in an internal combustion engine.
2. Description of the Prior Art
Knock detecting apparatus of the above type are known in the art in which a piezoelectric acceleration detector including a piezoelectric element is used to detect the knock-induced vibrations of an engine.
However, these known apparatus have the following disadvantages.
(1) Due to the use of a piezoelectric element of the essentially high-impedance type, its characteristics are subject to the effects of humidity and dirt and thus it is difficult to generate stably small trace-knock signals (due to large drift and vibration variations). Also, the piezoelectric characteristic of the element is determined by polarization, and thus due to the severe use conditions varying cyclically between the high and low temperature conditions the polarization tends to be lost gradually with the corresponding deterioration of the sensitivity. The decreased sensitivity results in a rise in the knock control level causing sometimes damages to the engine due to its knocking.
(2) It is desired that the detector is constructed inexpensively. However, since the piezoelectric element is a high-impedance device, its output signal must be amplified by means of an expensive amplifier of the high-impedance input type which is called as a charge amplifier. Also, since the high-impedance type amplifier tends to malfunction due to the effect of noise or the like caused by the ignition signals of the engine, it is necessary to use a detector of more expensive and complicated construction to overcome the deficiency.
(3) When used as a vehicle detector, the piezoelectric element has many disadvantages in terms of durability, cost and the like such as lower resistance to shock, tendency to crack and break and necessity to use cooling means or the like when the detector is used in high temperature conditions.